Supporting the teaching of Earth sciences at all levels
VIRTUAL EXPERIMENTS
EARTH SCIENCE TEACHERS’ ASSOCIATION
Experiment: Mike Tuke              Realised by: Julian & Phillipa Priddle                  Sponsor: PESGB

EARTHQUAKE SIMULATION

This animation shows the relationship between movement, resistance to movement and stress along a fault.  It also explains why the magnitudes of earthquakes along a given fault are so variable.

Resistance to movement

Resistance to movement depends on the roughness of the rock surfaces. If both sides of the fault are clay then that part of the fault will be able to move relatively easily.  On the other hand if both sides are granite it will take a much greater pressure to initiate the movement.  In these animations the resistance to movement and the initial stress levels for each section were randomly generated.

Stress

The stress increases slowly and continuously along the whole fault and on any large fault is due to plate tectonic movement.   A section of the fault will slip if the stress is greater than the resistance to movement.  When a section slips the stress in that section is considerably reduced and some of that stress is transferred to adjacent sections. In small earthquakes not enough stress is transferred to adjacent sections to cause them to slip. However in some cases the stress transferred will increase the stress in those adjacent sections so that it is greater than the resistance and further slip will happen.  This is how the slip extends itself.

This animation

There are 5 separate faults. On each animation a fault runs east-west through the centre of a map of a wooded area.  Each fault is divided into 15 sections and they are lettered from A to O.  The resistance to movement and stress for each section are shown as bars at the bottom of the page under the map of the fault.  The resistance to movement is the thinner grey bar on the left and the stress is shown by the thicker purple/yellow bar on the right.  At the beginning of a timeframe the stress in each section is increased by one unit.  The effects and ramifications of that increase are developed during that timeframe.  Movement will occur if in any section the stress value is greater than the resistance.   If there are no movements then pressing “next“ will move to the next timeframe.  If there is any movement then it will be shown by a yellow line along the fault above the section that moved. By pressing “next” again you will be able to see how the stress values change. The previous value for the stress in a section is shown by a green line across the bar.  These changes in stress may cause further movement in adjacent sections. Once the system has reached a stage when the stress value is lower than the resistance to movement in all sections then the total length of movement is shown by the yellow line and a magnitude value is shown above the map. Clicking on “next” will take you to the next timeframe. In each animation the resistance to movement and initial stress values in each section were given a random number.  The stress levels in each section increase by one unit for each timeframe.  The reduction of the stress in any section when it moves and the redistribution of that stress follow predetermined rules.   This shows how the stress levels change and earthquakes are generated but has the unfortunate consequence of there being rather too many earthquakes in some timeframes to be realistic.

Activities

Open the earthquake animations by clicking on the button below. 1. Examine the bar charts and try to predict which sections if any are going to slip.  Press “next” to see if you were correct in your choices.  Watch how the stress levels change.  Repeat the process by working your way through all the timeframes noting the different types of slip. 2. Find examples and record the earthquake activity  number and timeframe of each of the following: o A timeframe with no movement. o A single section moving. o A section moving and then causing one adjacent section to move. o A section moving and then causing several adjacent sections to move in turn to give a magnitude 4 earthquake. o A section moving and then causing several adjacent sections to move in turn resulting in a magnitude 5 earthquake. o Two or more adjacent sections slipping at the same time. 3. Record the magnitude values given at the end of each timeframe.  What do you notice about the frequency of each value.  Record or collect from other students the data from each of the five faults and add up the total number of the 2, 3, 4, and 5 magnitudes.   Plot the log of the frequency against the magnitude.
Earthquake animations